Report errors on duplicate export.

tests/language/duplicate_import_libc.dart

Index: tests/language/duplicate_import_libc.dart
diff --git a/tests/language/duplicate_import_libc.dart b/tests/language/duplicate_import_libc.dart
new file mode 100644
index 0000000000000000000000000000000000000000..d1f1b41cfe13f16b4c05709922deade56dc3c9ba
--- /dev/null
+++ b/tests/language/duplicate_import_libc.dart
@@ -0,0 +1,11 @@
+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library libc;
+
+var field;
+
+void method() {}
+
+class Class {}